Abstract. The hygroscopicity of an aerosol strongly influences its effects on
climate and, for smaller particles, atmospheric lifetime. While many
aerosol hygroscopicity measurements have been made at lower relative humidities
(RH) and under cloud formation conditions (RH>100%), relatively few
have been made at high RH (99 to 100%), where the Kelvin (curvature)
effect is comparable to the Raoult (solute) effect. We measured the size of
droplets at high RH that had formed on particles composed of one of
seven compounds with dry diameters between 0.1 and 0.5 μm. We report
the hygroscopicity of these compounds using a parameterization of the
Kelvin term, in addition to a standard parameterization (κ) of the
Raoult term. For inorganic compounds, hygroscopicity could reliably be
predicted using water activity data (measured in macroscopic
solutions) and assuming a surface tension of pure water. In contrast,
most organics exhibited a slight to mild increase in hygroscopicity
with droplet diameter. This trend was strongest for sodium dodecyl
sulfate (SDS), the most surface-active compound studied. The results
suggest that, for single-component aerosols at high RH, partitioning of
solute to the particle-air interface reduces particle hygroscopicity
by reducing the bulk solute concentration. This partitioning effect
is more important than the increase in hygroscopicity due to surface
tension reduction.
Furthermore, we found no evidence that micellization limits SDS activity
in micron-sized solution droplets, as observed in macroscopic
solutions. We conclude that while the high-RH hygroscopicity
of inorganic compounds can be reliably predicted using readily
available data, surface-activity parameters obtained from macroscopic
solutions with organic solutes may be inappropriate for calculations
involving micron-sized droplets.